Cold sores, also known as fever blisters, are tiny, clear, fluid-filled blisters that form around the mouth and are caused by the herpes simplex virus (usually type 1, or HSV-1) living inside your nerve tissue. Cold sores usually do not last longer than two weeks. However, the sores are highly contagious and tend to recur when the virus is reactivated by a trigger such as stress, sunlight, fever or illness.